Accidentally let slip a secret

At my school, each class is separated into different sets for ability There are 5 sets, with 1 being the best and 5 being the worst

For most subjects, I’m in set 3, which is slap bang in the middle. However, for science, I’m in set 4. Now you may be thinking “at least he’s not in the bottom set”

However, there are only 4 sets for science. I had my parents know that I’m in set 3 for most things, and set 4 for science.

However, I think I gave them the impression that there were still 5 sets. I was unaware that there was only 4 until recently though so it’s partially an error on my part.

I was struggling with my homework, asked my mom to help with it, she was confused about why I was struggling. I gave a snarky reply: “Well, I’m in the bottom set, what do you expect?” She let out a very audible gasp, I quickly backtracked saying I said setS, the plural (having a lisp came in handy here). She still seemed unamused though, despite the fact that she knew I was in set 4 from the beginning of the year.

I think that snarky comment I made ruined the illusion that I wasn’t in the bottom set, and now she’s probably going to yammer on and on about how I need to apply myself.

How can I tell her the truth about being in the bottom set? I don’t think she’ll take it too well, as I’ve basically been lying to her for 6 months, however I didn’t even know I was lying until like 3 weeks ago. Should I just pretend like the bottom set comment never happened or should I tell her the truth?